Pressure sensor generating a transduced signal with reduced ambient temperature dependence, and manufacturing method thereof

ABSTRACT

A pressure sensor designed to detect a value of ambient pressure of the environment external to the pressure sensor includes: a first substrate having a buried cavity and a membrane suspended over the buried cavity; a second substrate having a recess, hermetically coupled to the first substrate so that the recess defines a sealed cavity the internal pressure value of which provides a pressure-reference value; and a channel formed at least in part in the first substrate and configured to arrange the buried cavity in communication with the environment external to the pressure sensor. The membrane undergoes deflection as a function of a difference of pressure between the pressure-reference value in the sealed cavity and the ambient-pressure value in the buried cavity.

BACKGROUND Technical Field

The present disclosure relates to a pressure sensor in which thetransduced pressure signal has a reduced ambient temperature dependenceand to a manufacturing method thereof.

Description of the Related Art

As is known, a pressure transducer or sensor is a device that converts avariation of pressure into a variation of an electrical quantity (aresistance or a capacitance). In the case of a semiconductor sensor, thepressure variation is detected thanks to the presence of a membrane (ordiaphragm) of semiconductor material that overlies a cavity and is ableto undergo deflection in the presence of a force acting thereon.Deflection of the membrane is measured, for example, by piezoresistiveelements constrained to the membrane itself (i.e., on the basis of thecapacity of some materials to modify their resistivity as a function ofthe deflection to which they are subjected).

Piezoresistors are normally provided on the edge of the suspendedmembrane and/or are connected together in Wheatstone-bridgeconfiguration. Application of a pressure causes a deflection of themembrane, which in turn generates a variation of the offset voltage ofthe bridge. By detecting the variation of voltage with an appropriateelectronic circuit it is thus possible to obtain the desired pressureinformation.

Pressure sensors typically include a cavity in one side of the flexiblemembrane to enable deflection of the latter. This cavity forms apressure reference and is provided so that said pressure reference isvacuum pressure. In this way, the membrane is under the influence of anabsolute pressure. This type of pressure sensor finds wide applicationin the vacuum-technology industry, in space applications, and in all thesectors in which it is of interest to measure an atmospheric pressurewith respect to an absolute reference that is minimally dependent uponexternal environmental conditions (e.g., the working temperature of thesensor itself).

However, production of an absolute pressure sensor involveshigh-precision technological processes in order to couple the membraneperfectly on the cavity in vacuum conditions.

Currently, various solutions have been proposed for manufacture ofpressure sensors, amongst which: use of silicon-in-insulator (SOI)substrates; wet etching from the front (see for example U.S. Pat. No.4,766,666); wet etching from the back; and other methods still (see, forexample, U.S. Pat. No. 4,744,863).

In all of the known aforementioned solutions, use of semiconductortechnology for providing cavities underneath suspended structures andlayers involves processes that are complex, costly, and in some casesscarcely compatible with the current steps used in the semiconductorindustry for manufacture of integrated circuits.

FIG. 1 is a lateral sectional view of a pressure sensor 10 of a knowntype, which provides a solution to the problems set forth above.According to the embodiment of FIG. 1, a membrane 1 extends in one upperside 5 a of a wafer of semiconductor material 5, for example silicon.The membrane 1 is suspended over a buried cavity 2, which is formedaccording to the manufacturing method described in U.S. Pat. No.8,173,513.

The steps for obtaining the buried cavity 2, described in U.S. Pat. No.8,173,513, envisage a high-temperature processing of the wafer 5, in anenvironment at controlled pressure (but not vacuum pressure). Followingupon closing of the cavity 2 (completion of the formation of theoverlying membrane 1), the wafer 5 is then cooled, thus generating areduction of the pressure inside the buried cavity 2. However, duringuse of the sensor 10 thus obtained, an increase in the temperature ofthe environment in which the sensor 10 operates causes a consequentincrease in the pressure inside the buried cavity 2. A variation of theabsolute reference pressure provided by the pressure inside the buriedcavity 2 leads to a variation of the transduced output signal of thesensor 10 also in the absence of a variation of the ambient pressure tobe measured. These variations of the output signal may, at least inpart, be compensated for during a step of processing of the outputsignal but are, however, undesirable.

BRIEF SUMMARY

Some embodiments of the disclosure are a pressure sensor with reduceddependence upon the temperature of the transduced signal, and amanufacturing method thereof that will overcome the disadvantages of theknown solutions.

According to the present disclosure a pressure sensor and amanufacturing method thereof are provided.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For an understanding of the present disclosure preferred embodimentsthereof are now described, purely by way of non-limiting example, withreference to the attached drawings, wherein:

FIG. 1 shows a cross-section of a pressure sensor of a known type;

FIGS. 2 and 3 show in lateral sectional view and in top plan view,respectively, a pressure sensor according to one embodiment of thepresent disclosure;

FIGS. 4 and 5 show in lateral cross-sectional view and in a top planview, respectively, a pressure sensor according to a further embodimentof the present disclosure;

FIG. 6 shows in lateral sectional view a pressure sensor according to afurther embodiment of the present disclosure;

FIG. 7 shows in lateral sectional view a pressure sensor according to afurther embodiment of the present disclosure;

FIG. 8A-14 show, in lateral sectional view, manufacturing steps of thepressure sensor of FIGS. 2 and 3; and

FIG. 15 shows an intermediate manufacturing step for obtaining thepressure sensor of FIGS. 4 and 5.

DETAILED DESCRIPTION

FIG. 2 shows a lateral sectional view of a pressure transducer or sensor11, obtained in MEMS technology, according to one aspect of the presentdisclosure. The cross-section of FIG. 2 is represented in a system ofmutually orthogonal Cartesian axes X, Y, and Z, and is taken along aline of section shown in FIG. 3. The pressure sensor 11 comprises a bodyincluding a substrate 12, of semiconductor material, such as silicon,coupled to a cap 14, which is also of semiconductor material, such assilicon. The substrate 12 has a first face 12 a and a second face 12 b,opposite to one another, along the axis Z. A cavity 16 extends withinthe substrate 12, separated from the first face 12 a by a thin portionof the substrate 12, which forms a membrane 18 suspended over the cavity16. The membrane 18 has a thickness, along the axis Z, comprised between4 and 40 μm, for example 6 μm.

The cavity 16 has a thickness, along the axis Z, smaller than thethickness, along Z, of the substrate 12. In other words, the cavity 16extends buried within the substrate 12, between the first and secondfaces 12 a, 12 b. According to one embodiment, the cavity 16 has, in aview in the plane XY, a circular or polygonal shape, for example squarewith side of 350 μm. Along Z, the cavity 16 has a depth comprisedbetween 1 and 6 μm, for example 4 μm.

The cap 14 is coupled to the first face 12 a of the substrate 12 inperipheral regions of the membrane 18, by a coupling region 20. Sincethe coupling region 20 is arranged in peripheral portions of themembrane 18, during use the membrane 18 is free to undergo deflectionand not undergo interference caused by the presence of the couplingregion 20. The coupling region 20 extends along the entire perimeter ofthe membrane 18 and is, for example, of a glass-frit type. Other typesof bonding may be used, such as for example metal bonding (e.g.,gold-gold), eutectic bonding (e.g., Al—Ge).

The cap 14 has a recess 14′ directly facing the membrane 18. The recess14′ houses a getter layer 22. The getter layer 22 has the function ofgenerating, in use and when the recess 14′ is sealed (i.e., when the cap14 is coupled to the substrate 12 by the coupling region 20), areference pressure P_(REF) within the recess 14′ different from (inparticular, lower than) the pressure P_(A) present in the environmentoutside the recess 14′. Materials used as getter layer 22 are known andcomprise, for example, metals such as aluminum (Al), barium (Ba),zirconium (Zr), titanium (Ti), vanadium (V), iron (Fe), or correspondingmixtures or alloys, such as zirconium-aluminum, zirconium-vanadium-iron,zirconium-nickel, AND zirconium-cobalt (in particular, an alloy ofZr/Co/O). The getter layer 22 is, according to one embodiment, of anon-evaporable-getter (NEG) type, provided in the form of layer on theexposed surface of the recess 14′, in a manufacturing step prior to thestep of coupling of the cap 14 to the substrate 12. As is known, duringthe step of formation of the getter layer 22, the material of which thegetter layer 22 is made reacts with the surrounding air, causingformation of a passivating layer (typically of oxide or oxide/nitride)that coats the surface area of the getter layer 22 completely, renderingit inactive. Activation of the getter layer 22 occurs (following uponhermetic sealing of the recess 14′) by local activation in temperaturein order to remove the passivating layer that has formed on the surfaceof the getter layer 22. In this way, the getter layer 22 is activatedand operates in a known way by reacting with residual gases within therecess 14′ and enabling a reduction of the reference pressure P_(REF)with respect to the ambient pressure P_(A). The reference pressureP_(REF) represents the vacuum pressure.

It is evident that the getter layer 22 may be omitted in the case wherethe step of sealing of the recess 14′ takes place at controlledatmosphere and pressure. The extension of the recess 14′ along the axesX, Y, and Z is chosen so to generate, after coupling of the cap with thesubstrate 12, a reference cavity 24 with a volume comprised between1e-13 m³ and 50e-13 m³, for example 10e-13 m³. The pressure P_(REF)inside the reference cavity 24 is the reference pressure for measurementof the absolute pressure by the pressure sensor 11. It is consequentlyimportant for the coupling region 20 to seal the reference cavity 24hermetically, preventing any exchange with the external ambient pressureP_(A). In use, i.e., following upon activation of the getter layer 22 orfollowing upon the step of sealing of the recess 14′ should the sealingtake place at controlled atmosphere and pressure, the reference pressureP_(REF) inside the reference cavity 24 has a value of approximately 0mbar when measured at an ambient temperature of approximately 25° C.

The pressure inside the cavity 16 is, in use, equal to the ambientpressure P_(A) to be measured. For this purpose, the cavity 16 isfluidically coupled to the environment external to the pressure sensor11 so that its internal pressure stabilizes at the ambient pressureP_(A) (in this context, and in the following description, the fluidconsidered is air). For this purpose, according to one aspect of thepresent disclosure, the substrate 12 has one or more channels 26 (twochannels 26 are illustrated in FIG. 2, with a dashed line in so far asthey are not visible along the line of section of FIG. 3), which connectthe cavity 16 with the environment external to the pressure sensor 11.The channels 26 are thus through holes provided in the substrate 12.

According to the embodiment of FIG. 2, the channels 26 have: a mainextension along the axis X, for example of 100 μm; a thickness, along Z,equal to the thickness, along Z, of the cavity 16, for example of 4 μm;and a dimension along Y for example of 10 μm. The dimension along Y ofthe channels 26 is smaller than the corresponding dimension of thecavity 16 in order not to generate undesirable structural weaknesses ofthe substrate 12.

According to one embodiment, the dimension of the channels 26 along Y isnot constant, but is greater in the proximity of lateral faces 12 c, 12d of the substrate 12 (which is shaped substantially like a squeezedfunnel, or has a squeezed frustoconical shape, where the opening withsmaller area directly faces the cavity 16, and the opening with largerarea faces the outside of the pressure sensor 11). In this way, anypossible obstruction of the channels 26 by material deriving fromoutside the substrate 12, for example during a dicing step for formationof the dice that integrate the pressure sensor 11, is prevented.

The channels 16 are formed, for example, during the same steps ofcreation of the cavity 16 and using the same manufacturing process (forexample, the one described in U.S. Pat. No. 8,173,513).

In this way, the cavity 16 is fluidically connected with the externalenvironment, and the pressure inside the cavity 16 is the ambientpressure P_(A) to be measured.

FIG. 3 is a top plan view of the pressure sensor 11 of FIG. 2, in theplane XY. As may be more fully appreciated from FIG. 3, according tothis embodiment, four channels 26 are present that branch offsymmetrically from respective regions of the cavity 16 and connect thelatter with the outside of the sensor 11.

The membrane 18 further has one or more piezoresistive elements 28arranged in peripheral regions of the membrane 18 and facing the insideof the reference cavity 24. In other words, the piezoresistive elements28 are formed in regions of the face 12 a that, at the end of themanufacturing steps, are contained within the reference cavity 24. Thepiezoresistive elements 28 may be protected by a thin layer ofdielectric (e.g., silicon nitride with a thickness of approximately 0.5μm, or less) or else may face, or be exposed directly towards, theinside of the reference cavity 24. In this case, since the referencecavity functions as protection for the piezoresistive elements 28, theyare not subject to deterioration caused by atmospheric agents present inthe environment in which the pressure sensor 11 operates. Consequently,it is not necessary to provide a layer of protection of thepiezoresistive elements 28, with the advantage that these piezoresistiveelements 28 are effectively housed in surface portions of the membrane18, which are more subject to stress during use. The sensitivity of thepressure sensor 11 is thus improved.

According to one embodiment, the piezoresistive elements are provided asregions of a P type, formed by implantation of dopant atoms on the side12 a of the substrate 12, whereas the portion of the substrate 12 thatforms the membrane is of silicon with a doping of an N type. In FIG. 3four piezoresistors 28 are present, electrically connected together inWheatstone-bridge configuration. The interconnections between thepiezoresistors 28 (e.g., metal regions extending over an insulatinglayer), albeit present, are not represented in the figure exclusivelyfor simplicity of representation. As an alternative to what has beensaid, the piezoresistors 28 may be made polysilicon, for example with adoping of a P type, deposited on the membrane 18. A plurality of contactpads 30 extend in an area of the pressure sensor external to themembrane 18 and to the coupling region 20. The contact pads 30 are ofconductive material, such as metal, and form an interface for electricalconnection with the outside of the sensor, for example for acquiring thetransduced signal supplied at output by the Wheatstone-bridge circuitformed by the piezoresistors 28.

As is known, during the production processes, formed on a wafer ofsemiconductor material are a plurality of pressure sensors 11 of thetype illustrated in FIGS. 2 and 3. A final processing step envisagesdicing for forming dice, each of which houses a respective pressuresensor 11. In the case where the channels 26 are formed during the samestep of formation of the cavity 16, it may happen that, during dicing,one or more of the channels 26 is occluded by waste material derivingfrom the dicing step itself. This effect is, obviously, undesirable. Inorder to overcome this drawback, one embodiment of the presentdisclosure envisages that the channels for connecting the cavity 16 withthe outside are provided at the top, in the area of the cap 14. Thisembodiment is illustrated in FIGS. 4 and 5. FIG. 4 is, in particular, across-sectional view of the pressure sensor of FIG. 5, taken along theline of section V-V of FIG. 5.

With reference to FIG. 4, a pressure sensor 31 is illustrated in lateralsection. Elements of the pressure sensor 31 that are in common withthose of the pressure sensor 11 of FIG. 2 are designated by the samereference numbers and are not described herein any further. One or morechannels 32 (two channels 32 are represented with a dashed line in FIG.4, in so far as they are not visible along the line of section V-V),which have, in cross-sectional view in the plane XZ, the shape of an Lor a T turned upside down, extend in the substrate 12 starting from thecavity 16, in fluidic connection with the cavity 16. A first portion 32′of each channel 32 extends along the axis X as partial prolongation ofthe cavity 16 (the first portion 32′ is similar to the channels 26 ofFIGS. 2 and 3), whereas a second portion 32″ of each channel 32 extendsalong the axis Z starting from the first portion 32′ through the face 12a of the substrate 12, and completely through the cap 14. In this way,openings 36 are provided, fluidically connected with the cavity 16, onan upper surface 14 a of the cap 14, exposed towards the externalenvironment and thus at the pressure P_(A) to be measured. The secondportions 32″ of the channels 32 are formed, for example, by anisotropicetching such as deep reactive ion etching (DRIE), whereas the firstportions 32′ of the channels 32 are formed, for example, during the samestep of formation of the cavity 16 (for instance, according to theprocess described in U.S. Pat. No. 8,173,513), or using some othertechnique.

FIG. 5 shows, in a top plan view in the plane XY, the pressure sensor ofFIG. 4. As may be noted from FIG. 5, four openings 36 are present at thefront of the pressure sensor 31, formed laterally with respect to thecoupling region 20.

According to the embodiment of FIGS. 4 and 5, the second portions 32″ ofthe channels 32 extend outside the coupling region 20. According to afurther embodiment, illustrated in FIG. 6, a pressure sensor 39comprises second portions 32″ of the channels 32 that extend through thecoupling region 20. In this case, during the step of formation of thecoupling region 20, through openings 40 may be provided in portions ofthe coupling region 20 in which a respective second portion 32″ of arespective channel 32 will be formed.

According to a further embodiment, illustrated in FIG. 7, a pressuresensor 41 comprises channels 32 that extend exclusively in the substrate12 (and not also through the cap 14). In this case, openings 44 forfluidic access towards the cavity 16 are provided on the face 12 a ofthe substrate 12, laterally with respect to the coupling region 20 andoutside the reference cavity 24. According to this embodiment, formationof the channels 32 is carried out prior to the steps of formation of thecoupling regions 20 and of coupling between the substrate 12 and the cap14. In order to prevent the step of formation of the coupling region 20from causing an undesirable obstruction of the channels 32 thus formed,it is expedient to provide the channels 32 at a sufficient distance fromthe portions of the face 12 a in which the coupling region 20 is to beformed, for example at a distance, measured along the axis X, of 100 μmin the case of glass frit (in the case of use of some other bondingtechniques, for example metal bonding, this distance may be different,either smaller or greater). In order to prevent the material used forthe glass frit from expanding on the surface 12 a of the substrate 12 inan undesirable way, there may be provided containment regions forexample in the form of trenches (not illustrated) obtained by etchingselective portions of the surface 12 a, alongside the surface area inwhich the coupling region 20 is to be formed.

Described in what follows is a method for manufacturing the pressuresensor of FIGS. 2 and 3.

FIG. 8A is a cross-sectional view of a semiconductor wafer, made inparticular of monocrystalline silicon, during an initial step ofmanufacture of the pressure sensor 11. With reference to FIG. 8A, thesemiconductor wafer comprises a substrate 12 of an N type. Provided onthe surface of the substrate 12 is a resist mask 103. As may be seen inFIG. 8B (which shows a qualitative top plan view, not in scale, of thewafer of FIG. 8A), the mask 103 has a circular or generically polygonalsensor area 103′ (for example, square, as illustrated in FIG. 8B), andlikewise has elongated regions 103″ that depart from the sensor area103′ and define, as regards shape and dimensions, the channels 26 thatare to be formed. The mask 103 defines a honeycomb lattice (as may benoted more clearly from the enlarged portion of FIG. 8C), which presentsmask regions of a hexagonal shape arranged up close to one another.

Using the mask 103 (FIG. 9), etching of the substrate 12 is carried outto form a trench 106, which has a depth of some microns, for exampleapproximately 10 μm, and defines silicon columns 107 that are the sameas one another and have a shape corresponding to the shape of thehoneycomb regions defined by the mask 103. Likewise illustrated in FIG.9 (with a dashed line) are trenches 108 that will lead, in subsequentprocessing steps, to formation of the channels 26.

Next (FIG. 10), the mask 103 is removed and an epitaxial growth iscarried out in deoxidizing environment (typically, in atmosphere thatpresents a high concentration of hydrogen, preferably usingtrichlorosilane—SiHCl₃). Consequently, an epitaxial layer 110(hereinafter not distinguished from the substrate 12 and designated bythe same reference number 12), of an N type, grows above the siliconcolumns 107 and closes the trench 106 at the top, trapping the gaspresent therein (here, molecules of hydrogen—H₂). The thickness of theepitaxial layer 110 is of some microns, for example between 1 and 40 μm.

An annealing step is then carried out, for example for 30 minutes at1190° C. The annealing step causes (FIG. 11), in a per se known manner,a migration of the silicon atoms, which tend to move into the positionof lower energy. Consequently, in the area of the trenches 106 and 108,where the silicon columns are arranged close to one another, the siliconatoms migrate completely and form, respectively, the cavity 16, closedat the top by the membrane 18, and the channels 26, which are alsoclosed at the top by the silicon regions alongside the membrane 18.Owing to the presence of the cavity 16, the membrane 18 is flexible andmay undergo deflection.

Preferably, annealing is carried out in an H₂ atmosphere for preventingthe hydrogen present in the trench 106 from escaping through theepitaxial layer 110 outwards and for increasing the concentration ofhydrogen present in the cavity 16 and in the channels 26, in the casewhere the hydrogen trapped during the step of epitaxial growth were notsufficient. Alternatively, annealing may be carried out in nitrogenenvironment.

Next, in a way not illustrated in the figure, selective portions of themembrane 18 are doped via implantation of dopant species of a P type,for example boron, in order to provide the piezoresistive elements 28.The step of formation of piezoresistors in selective portions of amembrane, as likewise their Wheatstone-bridge connection, is per seknown and is thus not described herein any further.

If so desired, it is possible to integrate electronic components,constituting the control circuitry of the pressure sensor 11, and/orelectrical contact pads (e.g., the pads 30 of FIG. 3) within thesubstrate 12, in regions external to the membrane 18, in a per se knownmanner that does not form the subject of the present disclosure.

Then (FIG. 12), a wafer is laid, having a substrate 120, in which thecap 14 is to be provided. For this purpose, the substrate 120 is etchedon a face 14 b thereof to form the recess 14′, which extends in depthinto the substrate 120 for a thickness smaller than the thickness of thesubstrate 120 itself. The recess 14′ has dimensions, in the plane XY,such that, when the cap 14 is coupled to the substrate 12, the recess14′ is designed to surround the membrane 18 and the piezoresistors 28completely. Formed within the recess 14′, as already describedpreviously, is the getter layer 22, in a per se known manner.

Next, as illustrated in FIG. 13, the wafer comprising the substrate 12(in the processing step of FIG. 11) and the wafer comprising thesubstrate 120 (in the processing step of FIG. 12) are coupled together(step known as “wafer-to-wafer bonding”) to form the coupling regions20, for example by the glass-frit technique, so that the recess 14′faces, and completely surrounds, the membrane 18 (and the piezoresistors28 integrated in the membrane 18). Preferably, the coupling regions 20extend, on the face 12 a of the substrate 12, so to not projectlaterally (i.e., along X) with respect to the channels 26. In this way,a subsequent dicing step is carried out exclusively through supports ofsemiconductor material (the cap 14 and the substrate 12), and not alsothrough the coupling regions 20, which could be of a material notcompatible with the dicing tools used.

Finally (FIG. 14), the wafer is cut, along dicing lines 53, into dice,each containing a respective pressure sensor 11 thus obtained.

In order to manufacture the pressure sensor 31 of FIGS. 4 and 5, afterthe step of FIG. 13, step of FIG. 15 is envisaged, in which a DRIE stepis carried out (using an appropriate mask, not illustrated) on topregions of the cap 14 aligned, along Z, with respective portions of thechannels formed during the step of FIG. 9. The second portions 32″ ofthe channels for access to the cavity 16 are thus formed (which, in FIG.15, are represented with a dashed line in so far as they are not visiblealong the line of section VIII-VIII of FIG. 8B). Then the dicing step ofFIG. 14 is carried out.

Appropriate alignment markers may be envisaged, in a per se knownmanner, in order to facilitate identification of the top regions of thecap 14 aligned, along Z, with respective portions of the channels formedduring the step of FIG. 9.

The advantages that may be achieved with the pressure sensor describedemerge clearly from the foregoing description.

In particular, the transduced pressure signal, generated at output fromthe pressure sensor according to any one of the embodiments described,does not depend upon the residual pressure that is present in a buriedcavity. In fact, the reference pressure is now given by the pressurepresent in a cavity obtained by a process of coupling of substrates,which may be controlled with high precision (for example, using a getterlayer). The reference pressure, according to the present disclosure,does not vary, or varies minimally, with the temperature of theenvironment in which the pressure sensor works.

Furthermore, since the transducer elements (piezoresistors) face theinside of the reference cavity (which is hermetically closed), they areimmune from any impurities and atmospheric agents (dust, humidity, etc.)that might damage them or generate variations of the signal transducedthereby that are unforeseeable and may not be compensated for.

Finally, thanks to the manufacturing process described, the pressuresensor of silicon has a low cost and reduced dimensions, as well as animproved resistance to failure. In fact, since the cavity that receivesthe ambient pressure is of a buried type, to obtain it no further stepof coupling between substrates is required.

Finally, it is clear that numerous modifications and variations may bemade to the pressure sensor described and illustrated herein, all ofwhich fall within the scope of the inventive idea, as defined in theannexed claims.

For instance, the transduced signal generated as a function ofdeflection of the membrane 18 may be generated by a capacitive couplingof conductive regions of the membrane 18 with a fixed referenceelectrode. The conductive regions of the membrane 18 comprise, forexample, a thin metal layer, formed by deposition techniques of a knowntype. In this case, the piezoresistive elements are not necessary, andthe cavity 24 further houses the fixed reference electrode; the latterfaces the conductive regions of the membrane 18 so that the fixedreference electrode and the membrane 18 form respective plates of acapacitor. In use, deflection of the membrane causes a variation of thecapacitance of the capacitor thus formed. The measurement of saidvariation of capacitance may be correlated to the deflection of themembrane 18 which in turn may be correlated to the ambient pressureP_(A) acting thereon. The ambient pressure P_(A) may thus be measured.

Furthermore, the channel for connecting the cavity 16 with the externalenvironment may be formed for connecting the cavity 16 with the face 12b of the substrate 12, by providing fluidic access openings on said face12 b. The process of formation of said openings is similar to theprocess already described with reference to FIGS. 4 and 5 (e.g., DRIE ofthe substrate 12 starting from the face 12 b).

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification are incorporated herein by reference, in their entirety.Aspects of the embodiments can be modified, to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A method, comprising: forming, in a firstsemiconductor body, a buried cavity and a membrane suspended over theburied cavity; forming, in a second semiconductor body, a recess;coupling the second semiconductor body to the first semiconductor bodyso that the recess faces the membrane, thus defining a sealed cavityhaving an internal pressure value that provides a pressure-referencevalue; and forming a channel at least in part in the first semiconductorbody and in fluidic communication with the buried cavity.
 2. The methodaccording to claim 1, further comprising forming a transducer assemblyin a surface region of the membrane facing inside of the sealed cavity,the transducer assembly being configured to generate a transducedelectrical signal as a function of a deflection of the membrane.
 3. Themethod according to claim 1, wherein forming the buried cavity andforming the channel in the first semiconductor body are performedsimultaneously.
 4. The method according to claim 1, wherein forming thechannel comprises: etching the first semiconductor body as partialprolongation of the buried cavity, in a same plane as the buried cavity;and cutting the first semiconductor body for exposing the channel at aside wall, orthogonal to the plane, of the first semiconductor body. 5.The method according to claim 1, wherein forming the channel comprises:etching the first semiconductor body as partial prolongation of theburied cavity in a first direction belonging to a plane of the buriedcavity, to form a first subchannel; and etching the first semiconductorbody in a second direction orthogonal to the first direction, forconnecting the first subchannel fluidically with a side of the firstsemiconductor body exposed towards an external environment, to form asecond subchannel.
 6. The method according to claim 5, whereinmanufacturing includes: forming a coupling region that surrounds themembrane completely; and hermetically coupling together the first andsecond semiconductor bodies via the coupling region, wherein forming thesecond subchannel comprises etching the first semiconductor body outsidethe coupling region.
 7. The method according to claim 1, wherein formingthe channel comprises: etching the first semiconductor body as partialprolongation of the buried cavity, along a first direction belonging toa plane of the buried cavity, to form a first subchannel; etching thefirst semiconductor body along a second direction orthogonal to thefirst direction, for connecting the first subchannel fluidically with aside of the first semiconductor body facing the second semiconductorbody, to form a second subchannel; and etching the second semiconductorbody to form a through hole fluidically connected to the buried cavityvia the first and second subchannels.
 8. The method according to claim7, wherein manufacturing includes: forming a coupling region thatsurrounds the membrane completely; forming a through hole in thecoupling region; and hermetically coupling together the first and secondsemiconductor bodies via the coupling region, wherein forming the secondsubchannel comprises etching the first and second semiconductor bodiesin an area corresponding to the through hole of the coupling region. 9.The method according to claim 1 wherein forming the membrane and theburied cavity comprise: etching first trenches in the firstsemiconductor body, the first trenches delimiting between them firstwalls of semiconductor material; growing epitaxially, starting from thefirst walls, a closing layer of semiconductor material, said closinglayer closing the first trenches at the top to form the membrane; andcarrying out a thermal treatment such as to cause migration of thesemiconductor material of the first walls and forming the buried cavity.10. The method according to claim 9, wherein: forming the channelcomprises etching second trenches in the first semiconductor body, aspartial prolongation of the first trenches, the second trenchesdelimiting between them second walls of semiconductor material; growingthe closing layer epitaxially further comprises closing the secondtrenches at the tops of the second trenches; and carrying out thethermal treatment further comprises causing migration of thesemiconductor material of the second walls and forming the channel. 11.A method, comprising: forming a first group of discrete recesses on afirst surface of a first substrate of a first material, the first groupof discrete recesses arranged within a first area of the first surfaceand being separated from one another by a first substrate column;forming a layer over the first surface and covering the first group ofdiscrete recesses and the first substrate column; after the forming thelayer, forming a buried cavity in the first substrate by annealing thefirst substrate such that the first substrate column at least partiallydissolves due to atoms of the first material migrating; forming a recesson a second surface of a second substrate, the recess having an openingat the second surface, the opening having a size that is larger than thefirst area; coupling the second surface of the second substrate with thefirst surface of the first substrate such that the opening of the recessis fully overlapped by the first substrate and such that the opening ofthe recess overlaps the first area; and forming a channel at least inpart in the first substrate and in fluidic communication with the buriedcavity.
 12. The method of claim 11, wherein the forming the layer isconducted in a deoxidizing environment.
 13. The method of claim 12,wherein the deoxidizing environment includes hydrogen.
 14. The method ofclaim 13, wherein the forming the layer traps the hydrogen of thedeoxidizing environment within the first group of discrete recessescovered by the layer.
 15. The method of claim 14, wherein the annealingis conducted in a hydrogen containing environment.
 16. The method ofclaim 11, wherein the layer includes a same first material as the firstsubstrate.
 17. The method of claim 11, further comprising: forming asecond group of discrete recesses on a second area of the first surfacethat is adjacent to the first area, the second group of discreterecesses being separated from one another by a second substrate column,the second group of discrete recesses being separated from one or morediscrete recess in the first group of discrete recesses by a thirdsubstrate column, and wherein the annealing the first substrate at leastpartially dissolves the second substrate column and the third substratecolumn such that a first channel is formed through the second group ofrecesses which communicates with a buried space formed through the firstsubstrate column at least partially dissolving.
 18. The method of claim17, further comprising forming a second channel through the secondsubstrate and connecting to the first channel.
 19. A method, comprising:coupling a first substrate with a second substrate, the first substratehaving a buried cavity underneath a membrane on a first surface of thefirst substrate, the buried cavity being fully encapsulated, the secondsubstrate having a recess opening only through a second surface of thesecond substrate, the first surface facing the second surface, and therecess fully encapsulated at least partially by the first substrate; andafter the coupling, opening a channel in fluidic communication with theburied cavity.
 20. The method of claim 19, further comprising forming agetter layer on a bottom surface of the cavity before the coupling.